Variable speed/constant frequency (VSCF) electric power generating systems for use on aircraft are comprised of at least one source of electrical energy which converts the variable speed mechanical rotational energy of the engines to electrical energy, and at least one power conversion device which converts the variable frequency electrical energy output from the source to a constant frequency/constant voltage output for use by the loads on the aircraft, such as lighting, radar systems, flight control computers, and so forth. To deliver this constant frequency/constant voltage electrical energy to all of these various loads, feeders are routed from the source on the engine, which is located on the wing, to the power conversion device, which is located in a power distribution center located somewhere in the body of the aircraft. On a large commercial aircraft, the distance between these two locations is often quite far.
As with any distribution system, it is important to know that what is being placed into the system, to be carried via the feeders in this case, is being delivered to its intended destination. It becomes critical in the case of the electric power generation system on an aircraft for two reasons: 1) the probability of a short circuit developing somewhere along the feeders is high because of the changing and harsh environments, including vibrational levels, temperature and pressurization, through which they are routed; and 2) the damage which can occur as a result of an electrical short circuit of a primary power feeder is so great, potentially leading to fire or explosion.
The system of protection utilized on aircraft in the past for the detection and isolation of these potentially dangerous short circuits is shown in FIG. 1. In this system a current sensing means, as represented by current transformers 6, 8, 10, is placed at the source 17 of electrical energy to monitor the flow of electric current from the source 17 into the feeders 12, 14, 16. Another current sensing means, as represented by current transformers 18, 20, 22 is then placed at the power conversion device 31 in the load distribution center to monitor the flow of electric current out of the feeders. The current transformers 6 and 18, 8 and 22, 10 and 20 operate as pairs, one pair for each phase in a multi-phase system.
The operation of each pair set is identical, and, therefore, only one pair will be discussed.
The orientation of the current transformers (CTs) are such that, during normal no-fault operation, the proportional output current signal from the source current transformer 6 flows through resistor R.sub.2 in a direction 180.degree. out of phase from that originating from the conversion device current transformer 18. The resultant voltage, as measured across resistor R.sub.2, provides an indication of the fault, or differential, current. Under normal no-fault conditions the magnitude of the current sensed at both ends of the feeder are the same, and the voltage across resistor R.sub.2 is zero. During fault conditions some or all of the current generated by the source is supplied to the fault, and the proportional output current signals no longer cancel each other across resistor R.sub.2. The resulting voltage amplitude is monitored by circuit means 24 which generates a control output if the differential current exceeds a predetermined threshold. If the control signal is present beyond a predetermined time interval as measured by timing means 26, an output protection signal is generated indicating that a true differential current fault exists.
The problem with the protection system of the prior art is that the circuitry as discussed above must be duplicated for each phase of the multi-phase system. This adds weight and cost to the system, as well as reducing the reliability, due to the increased parts count. Another problem associated with the prior art is that the source and conversion device current transformers must be closely matched to ensure that a voltage potential is not generated when there is no fault within the system. This problem may become significant during overload and shock load conditions when the source is very hot and the conversion device is cool. Also, since the source and conversion device current transformers are physically located in different environments, electromagnetic interference (EMI) and high intensity radiated fields (HIRF) will affect the proportional output current signals differently, thus potentially creating a voltage potential across resistor R.sub.2 indicating a fault when, in fact, no fault exists.
The present invention is directed to overcoming one or more of the above problems by recognizing that the power conversion device exhibits essentially balanced load characteristics under normal no-fault operation. That is to say that normally the current drawn from each phase of a multi-phase source of electrical energy is equal, regardless of the balance of the actual loading of the power conversion device by the aircraft loads supplied thereby.